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Hours after a presidential commission raised new questions about the safety of the cement mixture intended to temporarily seal BP's Macondo exploration well, oil field service giant Halliburton said the final version of the mixture did not undergo a foam-stability test.

The presidential commission investigating the mammoth oil spill said Thursday that the cement mixture repeatedly failed lab tests before the April 20 blowout.

Halliburton later issued its own statement, dismissing some of those lab tests as preliminary or irrelevant but also adding - without explanation - that the cement mixture ultimately used on the well did not undergo the complete battery of safety tests.

As early as February, Halliburton was getting poor results in lab tests of the recipe for the cement it was planning to use, according to evidence collected by the National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling.

Three separate tests suggested that the mixture would be "unstable," according to a commission staff letter released Thursday. Halliburton notified BP by e-mail about only one of the tests before the well explosion, according to the commission. The two companies went ahead with the cementing job anyway. Its failure became the first in a cascade of factors leading to the accident.

The results of a fourth Halliburton test - the only one indicating that the cement slurry might have been able to contain the high-pressure pool of oil and gas at the bottom of the Macondo well - were not available until the night of April 19 at the earliest and perhaps not until after the cement was poured, the commission staff said.

The oil spill commission is sifting through the events leading to the April 20 explosion, which killed 11 workers, sank the Deepwater Horizon and triggered a massive oil spill in the Gulf of Mexico. The cement at the bottom of the exploratory well was supposed to have provided a seal until a production facility could be built.

The reason for the cement job's failure has been a matter of dispute for months. Halliburton has pointed at BP; BP has challenged Halliburton. Experts still differ.

Halliburton late Thursday night issued a statement disputing the commission staff's letter, calling the February tests "preliminary" and saying that "final well conditions were not known at that time." Halliburton added that the first of the April tests was "irrelevant" because of errors at the lab, and it said that, contrary to the commission letter, BP was informed of that test.

The news spooked shareholders; Halliburton's stock closed at $31.68 a share, down nearly 8 percent. BP said it had no comment.

The commission letter reiterated that the cement was just one contributor to the disaster. "Cementing wells is a complex endeavor, and industry experts inform us that cementing failures are not uncommon even in the best of circumstances," the commission letter says.

But the new details call into question whether Halliburton's recipe - which mixed nitrogen and other additives with ordinary cement to create a foamy mixture - was the right one.